In an engine-driven a-c generator, the frequency generated is dependent on the revolution of the engine. In order to stably produce an a-c voltage of a predetermined frequency, or a commercial frequency of 50 Hz or 60 Hz, for example, without being affected by changes in engine revolution, the a-c voltage generated by an a-c generator is converted into d-c voltage, which is in turn converted into an a-c voltage again using an inverter circuit.
An inverter circuit driven by pulse-width modulated (PWM) signals is usually used to convert a d-c power into an a-c power having a commercial frequency. Switching, elements of the inverter circuits are apt to be subjected to overcurrent, and are generally equipped with an overcurrent protective device.
The overcurrent protective device of the prior art detects, with a detecting resistor, the load current flowing in a switching element, and turns off the switching element when the detected load current exceeds a predetermined overcurrent setting.
As a result, this on-off operation is continued in a self-oscillation fashion so long as loading conditions remain unchanged. If the load current is of a value exceeding the dielectric strength of the switching element, this could destroy the switching element.
There is a method in which the inverter circuit is cut off as soon as an overcurrent is detected, and thereafter kept from being reset unless a reset signal is intentionally activated. This method, however, is not suitable for such a load like incandescent lamps because the inverter circuit could be kept cut off if used on a load, such as incandescent lamps where a current several to several dozens times as large as the rated current flows at the time of lighting.
As a power distribution system for distributing power to the engine-driven power generator, a technique disclosed in Japanese Patent Application No. 292111/1985 is publicly known.
In the power distribution system, FETs and other elements are used as a bridge-type inverter, step-down type chopper-type regulator and other switching means.
Since these elements are voltage-driven type elements, the capacity of the drive circuit can be reduced. The fact that gate voltage is applied to the source terminal, however, poses no problems if the source terminal is common to the earth, but requires an independent drive-circuit power source if the source terminal is not common to the earth.
For example, FIG. 7 is a circuit diagram of a bridge-type inverter, and FIG. 8 is that of a chopper-type regulator.
In the figures, reference numeral 305 refers to a load; 306 to a d-c power supply; and 311 through 315 to FETs, respectively.
The source terminals of the transistors FET311 and FET312 shown in FIG. 7 are connected to the load 305, and the source terminal of the transistor FET315 shown in FIG. 8 is also connected to a diode, coil, capacitor, load, etc. So, both do not form grounded-source circuits, and an independent power source for gate pulses is required with respect to the source terminal of each element. With such element configurations, therefore, an insulating transformer is used for the power circuit, or an insulating-type d-c/d-c converter is required. This may make the size of the power source unwantedly large.
FIG. 9 is a circuit diagram of a bridge-type inverter in which part of the circuit shown in FIG. 7 was replaced with a bipolar transistor, and FIG. 10 is a circuit diagram of a chopper-type regulator in which part of the circuit shown in FIG. 8 was replaced with a bipolar transistor. In the figures, reference numeral 316 through 324 refer to bipolar transistors. If the aforementioned power source means cannot be employed due to limitations in equipment size, circuits have to be constructed with bipolar transistors and other means, as shown in FIGS. 9 and 10.
Consequently, additional power is required in the drive circuit for bipolar transistors, and switching frequency cannot be set a high level. That is, advantages of FETs cannot be enjoyed.
As described above, if FETs are used as control elements, independent power sources are required, making the size of the power supply unwantedly large. On the other hand, if bipolar transistors are used as control elements, a large capacity of power source is required and switching frequency cannot be set at a high level.
In engine-driven generators, rotating-field type synchronous motors are generally used. In rotating-field type synchronous motors of an output of several hundred watts to several kilowatts, a stator 402 having as many as 36 slots 401, as shown in FIG. 13, if often used.
In the manufacture of engine-driven generators, the need for simplifying windings by reducing the number of slots of the stator is strongly felt to facilitate automation of wire winding operation, improve product quality, and reduce manufacturing manhours.
There may arise a problem in performance, however, merely reducing the number of slots 401. FIG. 14 shows the waveform of no-load voltage for a synchronous motor having 36 slots, and FIG. 15 shows the waveform of no-load voltage for that having 18 slots. As is evident from FIG. 15, slot ripples in the voltage waveform may pose a problem.
In portable type engine-driven generators, it is desired that power of a quality close to commercial power in terms of characteristics, and there is a conflict between reducing the number of slots 401 and reducing ripples.
There can be a means for improving slot ripples by skewing the rotor slots, for example, but it is difficult to substantially reduce the number of slots since there are limitations in improving ripples in terms of manufacturing process.